Alarm system



2 sheets-sheet 1 P. LAAKMANN ALARM SYSTEM un l Jne 15, 1965 Filed May 25. 1960 IIIIy 20T-.15km ...(mFZmU O... m

June l5., 1965 P. LAAKMANN ALARM SYSTEM Filed May 25. 1960 2 shams-sheet 2 United States Patent Olice li Patented June l5, i965 3,189,8S4 ALARM' SYSTEM Peter Laahmann, Staten island, NX., assigner to American District Teiegraph Company, Jersey City, Nui., a corporation of New Jersey Filed May 25, 196), Ser. No. 31,6@ 5 lCiaims. (Ci. S40-25d) The present invention reiates to electrical protection systems, and more particularly to electrical protection systems of the type which are sensitive to changes in capacity.

Electrical protection systems in which changes in capacity produced by the approach of an intruder to a protected area produce an alarm signal have been in use` for many years and have afforded excellent protection of property. An example of such a system is the one described in Lindsay and Woloschak United States Patent 2,343,987, granted March 14, 1944. The use of these systems has been limited to some extent by a tendency to register spurious alarms under adverse ambient conditions, eg., humid weather and exposure of the protected object or area to sun radiation.

The present invention is concerned principally with providing an improved electrical protection system of the capacity type and in which superior operating characteristics are achieved than with systems heretofore available. In general, it may be said that no electrical protection sysem is perfect and that any such system represents a compromise among several conliicting factors, chiefly sensitivity, reliability, stability, cost and freedom from spurious alarms. The system of the present invention provides a compromise among these factors yielding a better operating and more efficient system than those heretofore available.

The principal object of the present invention has been the provision of a novel and improved electrical protection` system of the capacity type.

More particularly, it has been an object of the invention to provide a novel and improved system of the above type which is relatively simple in construction and low in cost but which afford-s a high degree of sensitivity to the approach of an intruder and a low incidence of spurious alarms even under adverse ambient conditions.

` Another object of the invention has been the provision of a capacity type etectrical protection system adapted especially for use in central station service and in which operating power may be supplied over the central station signalling conductors.

A further object of the invention has been the provision of a system of the above type which will notregister an alarm upon a momentary interruption of power.

Still another object of the invention has been the provision of a capacity type electrical protection system which affords both rate of change and absolute limit types of operations.

Another object of the invention has been the provision of a capacity type electrical protection system in which simultaneous protection can be provided for a plurality of objects or areas having different system sensitivity requirements.

Still another object of the invention has been the provision of a system of the above type in which a plurality of objects such as safes and filing cabinets may be protected and in which closure operated contacts are provided on each object closure to complete the capacitive circuit.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

' The invention will now be described in greater detail in connection with the appended drawings, in which:

`FIG. 1 and FIG, 2, when joined along the line 2"-2, form a schematic circuit diagram of one for-m of system in accordance with the invention.

Protected 0b jects The invention Will be described in connection with the protection of objects such as safes, vaults and file cabinets, since it is in this field that the invention is expected to iind its greatest utility. However, the principles of the invention are also applicable to other protection situations in which variations in capacity can be utilized to provide an alarm signal indication. The invention will also be described in connection with a central station connection, since such a connection, Where available, is generally recognized to provide maximum protection. However, it will be apparent that the invention may be used to provide an alarm in or about the protected premises and/ or at any other desired place, local or remote.

Referring now to the drawings, there are shown at l@ and 1l metal objects to be protected against tampering or opening during periods of time in which protection is to be afforded. In a mercantile or industrial establishment these periods are usually nights and week-ends. The object 1t) might be a relatively heavy steel safe, while the object 11 might be a conventional steel filing cabinet. Both the safe iti and cabinet 1i are provided with one or more door (or drawer) switches arranged to be closed when the door (or drawer) is closed and open when the door (or drawer) is open. Where either the safe or cabinet `has more than one `such switch, they will be connected in Series. The contacts of file cabinet switch 12 tor the end contacts of series connected switches) are connected to terminals Tl and T2, respectively, through a shielded cable 13, the shielding of which is grounded by connection to terminal T3. Terminal T3 is connected to ground through a conductor 2l. The contacts of safe door switch l5 are similarly connected to terminals T4 and T5 through a shielded cable i6, the shielding of which is connected to terminal T3. One terminal of each of switches l2 and i5 is insulated from the associated safe or cabinet, While the other terminal thereof is electrically connected to the safe or cabinet.

Both the safe it) and the cabinet il should be electrically insulated from ground. Each has a capacitance to ground dependent largely on its surface area and location relative to grounded objects and the earth itself. However, to provide increased normal capacitance, it is desirable, but not essential, that each be located above a grounding screen, as indicated at 17 and i8. When the safe or cabinet is located adjacent a wall beyond which persons or large animals may be moving, it is desirable that capacitance variations caused by such movement be shielded, as by providing a grounded metal screen behind the object, eg., the metal screen i9 shown behind the cabinet Il.

Electronic circuit A capacitor 2t? is coupled between terminal T1 and ground, ground potential being supplied through a conductor 2l. Terminal T2 is connected to one end of a secondary winding 22 of a transformer T Fi. Terminal T4 is connected to the other end of winding 22. Terminal T5 is connected to one end of another secondary winding 23 of transformer TF1. The other end of winding 23 is connected to a terminal T6 which in turn is connected to a terminal T7. The terminal T7 is connected to one end of a secondary winding 24 of transformer TF1, the other end of winding 24 being connected to a terminal T. Terminal T8 is connected to ground potential through conductor 2l. Should there be another object to be protected, it may be connected to the terminals To and T7 in place aiaasaa of the strap shown, the connection being effected in the same way as shown for the objects 1f? and 11.

One end of primary winding 25 of transformer TF1 is coupled to a point 26 through the coil of a relay Y and an adjustable resistor 27, the latter being termed the night resistor. A capacitor 28 is normally connected in parallel with the coil of relay Y through back Contact 29 and armature 3d of relay Y. Point 26 is supplied With positive potential from the high side of the central station line 3i through an adjustable line resistor 32, protective circuits formed by cabinet lining 33 and door and tamper switch 34, high side fixed attachments 35, an on-off switch 36, high side movable attachments 37 and a bridge rectifier 33. scribed in greater detail hereinafter.

A fixed intermediate tap on primary Winding 25 is coupled to the emitter of a transistor 39 through a resistor 40. The transistor 39 might be, for example, of the 2N44 type. The other end of primary winding 25 is coupled to the base of transistor 39 through la capacitor 4l. A resistor 42 is coupled between the base and collector of transistor 35i'.

The collector of transistor 39 is connected to the low side 43 of the central station line through rectifier bridge 38, low side movable attachments 44, low side fixed attachments 45 and bell contacts 46. The low side of the central station line is locally grounded through the ground screens 17, 18 and 19 located adjacent the safe 1t? and the file cabinet 11. A ground return connection to the central station may be provided in place of the conductor 43.

The transistor 39 is connected as a radio frequency oscillator, the oscillator circuit including transformer TF 1, the capacities to ground afforded by safe 1f) and cabinet 1l, the c-apacity to ground afforded by fixed capacitor Ztl and the capacity to ground afforded by a variable capacitor 47 connected in parallel with the capacitor 20; It will be evident -that the capacitors 20 and 47 and the capacity to ground of the cabinet 11 are effectively connected in parallel with each other and act between one end of the secondary winding of transformer TF1 and ground. The capacity to ground afforded by the safe acts between an intermediate point on this secondary Winding and ground.

Transformer TF1 is provided with a link winding 48 which supplies a portion of the high frequency energy output of the oscillator circuit to a series resonant circuit formed by a coil TF2 and a capacitor 49.

An intermediate tap on the coil TF2 is coupled to the base of a transistor 50 (which might be of the 2N167 type) through a rectifier 51 and the parallel combination of a rectifier S2 and a capacitor 53. Rectifiers 51 and 52, which are oppositely poled, might be of the 1N307 and 1N461 types, respectively.

Parallel connected resistor 54 and capacitor 55 serve to couple the junction of rectifiers 51 and 52 to an intermediate potential point formed by the junction of resistors 56 and 57. Resistors 56 and S7 form a voltage divider connected between the high and low sides of the power supply, the latter being designated conductors 58 and S9, respectively. Capacitors 60 and 61 are connected so as to shunt the resistors S6 and 57, respectively, and act to prevent any alternating Voltage being developed across these resistors. A rectifier 62, which might be of the 1N306 type, is connected between the base of transistor 5d and low (ground) side conductor 59.

The emitter of transistor 5f) is coupled to conductor 59 through a resistor 63, while the collector thereof is coupled to high side conductor 5S through the main coil of a relay X. The main coil of relay X is preferably of the high impedance type and might have, for example, an impedance of 16,990 ohms. In comparison, the relay Y might have an impedance of l0() ohms. A rectifier 6d, which might be of the 1N461 type, is connected in parallel with the main coil of relay X and acts to make relay X The various elements in this circuit will be dedslow to release. A capacitor 64' is connected in parallel with rectifier 64. Y

The radio frequency signal transferred to the resonant circuit TR2-49 from transformer TF1 is rectified by rectifier 51 and produces a DE. Voltage across resistor 54 and capacitor 55, which voltage may conveniently be termed the detector voltage. The magnitude of the detector voltage will depend, inter alia, upon the relationship between the signal frequency and the resonant frequency of the resonant circuit, or, in other words, upon the operating point on the resonance curve of the resonant circuit. The detector voltage will be a maximum when the signal and resonant frequencies coincide.

While a series resonant circuit is shown, the system may be operated with a parallel resonant circuit. In this case the voltage across the entire resonant circuit may be used as the source of the detector voltage. With a series Vreso- Y nant circuit, the voltage across either the capacitiveA or inductive branches may be used as the source of detector voltage or, as shown, a portion of the voltage across one of these branches. The current through a series resonant circuit may be used as the measure of the difference between the oscillator frequency and the resonant frequency. It is, of course, this difference and its rate of change which is the quantity measured to initiate an alarm. Where current through a series resonant circuit is to be measured, this current may be passed through a resistor included in the resonant circuit and the voltage drop across the resistor will be proportional to the current and may be rectified to produce the detector voltage.

The oscillator circuit may be arranged to produce an output signal in transformer TF1 of any convenient radio frequency, e.g., 50 kc. The actual oscillator frequency is adjustable by varying the capacity afforded by capacitor 47. For normal operation, the capacitor 47 is adjusted, with regard to the capacities afforded by safe 10 and cabinet 11, so that the oscillator frequency is somewhat different than the resonant frequency of the resonant circuit T 122-49. Preferably, the oscillator frequency is somewhat less than the resonant frequency s0 that an increase in the oscillator circuit capacity, such as will be caused by the approach of an intruder to the safe 10 or the cabinet 11, and the resulting decrease in oscillator frequency will result in a decrease in the detector voltage. The detuning provided under normal conditions by adjustment 0f capacitor 47 serves to place the normal operating point on a steep portion of the tuning curve of the resonant circuit, thus insuring a sensitive response to changes in oscillator frequency caused by motion of an intruder.

By way of example, if the signal frequency and resonant frequencies were to coincide, the detector voltage might be 20 Volts. The capacitor 47 might then be adjusted to decrease the signal frequency sufficiently that the detector voltage, under normal conditions, would be 10 volts. By normal conditions is meant that the capacities to ground of the safe 10 and the cabinet 11 have values indicating that there is no intruder within predetermined distances of either.

Under normal conditions the detector voltage will charge the capacitor 53. This capacitor is preferably relatively large, e.g., 500 microfarads. As mentioned previously, the approach of one or more intruders to the safe 1f! and/or the cabinet 11 will increase the capacity of the oscillator frequency determining circuit, lowering the signal (oscillator) frequency. But lowering the` signal frequency will in turn result in a decrease in the detector voltage.

Lowering of the detector voltage results in capacitor 53 discharging since this capacitor is normally maintained fully charged by the normal detector voltage. The amount of the discharge will depend uponthe drop in detector voltage experienced and will be large for large increases in capacity and small for small increases in capacity, e.g., such as might result from approach of a. small animal to the safe 10.

Rectifier 62, which is included in the charging circuit -for capacitor 53, will not pass the discharge current. Hence the discharge current of capacitor 53 flows into the base of transistor 50, causing an amplified current to iiow in the transistor collector circuit. This amplified current flows through the main winding of relay X. If the amplified current is sufficiently great, relay X will become energized.

Energization of relay X will close armature 65 and front contact 6d, thereby transmitting an alarm signal in a manner to be described hereinafter. The circuit is arranged so that the capacity change resulting from the approach of a small animal to the safe 10 or cabinet 11 will not result in sufiicient discharge current flow from capacitor 53 to produce energization of relay X. However, approach of a human being to either the safe or cabinet or both will result in sufficient capacity change so that the discharge current of capacitor 53 will be adequate to energize relay X.

lt will be appreciated that the discharge current of capacitor 53 is not only a function of the magnitude of the capacity change but also of the rate of capacity change. Thus if an intruder were to approach the safe 10 sullciently slowly, the discharge current flowing from capacitor 53 might never be adequate to result in energization of relay X even though the total capacity change resulting from the slow approach would be the same as that resulting from a normal approach. That is to say, the loss of charge of capacitor 53 will depend only on the capacity change in the frequency determining circuit, but the discharge current will be a function of both the capacity change and the rate at which such change takes place. The feature of not responding to very slow changes in capacity is undesirable from a sensitivity standpoint but is necessary to guard against spurious alarms resulting from changes in ambient conditions, eg., temperature and humidity. Changes in ambient conditions may result in capacity changes and hence will produce variations in detector voltage which might produce an alarm if the system were to respond to such changes in capacity occurring over a relatively long time interval.

An important aspect of the invention is concerned with preventing defeat of the system by an intruder'irrespective of how slowly he may move. For this purpose resistors 56 and 57, acting as a voltage divider, provide a small voltage across resistor S4 which is opposite in polarity to the detector voltage. This small voltage may conveniently be termed a supervisory voltage. With the arrangement shown, this supervisory voltage would be positive since the detector voltage would be negative. With a normal detector voltage of r- 10 volts, the supervisory voltage might be +5 volts.

If the detector voltage drops below the supervisory voltage, the potential across rectifier S2 will reverse, allowing current to flow through the base circuit of transistor Sl) and causing relay X to operate. Thus while the detector voltage acting to charge or discharge capacitor 53 provides a rate of change sensitivity, the supervisory voltage provides an absolute change sensitivity, i.e., an alarm response to attainment of a particular capacity value irrespective of the time required for the change. The` supervisory voltage may be selected by appropriate selection of voltage divider resistors 56 and 57 to provide the absolute change sensitivity at any desired threshold of capacity value, rendering ineffectual an attempt to defeat the system by slow movement but without increasing the hazard of a false alarm so long as the supervisory voltage is below a value which may be achieved by the detector voltage under adverse ambient conditions.

Rectifier 62 conducts only when capacitor 53 is charging, but this rectifier provides a relatively short charging time constant so that fast restoration of system sensitivity is achieved. However, the system cannot be used unless a minimum detector voltage is present since otherwise the supervisoryvoltage will keep the system in alarm. This also provides supervision of closure of the safe and cabinet doors. Thus if one of the doors is not closed the corresponding switch 12 or 15 will be open, breaking 4the capacity circuit so that the frequency will rise to a high value such that the detector voltage will be zero and the system cannot be reset from the central station.

Operating power for the electronic components of the system may be provided locally, but considerable advantage is achieved by providing this power over the central station line. In particular, protection is maintained despite local power failures. Where local power is used, if this power fails, the system will not be able to detect intruders and the design, for safety, should provide an alarm signal upon power failure since the power failure may be a deliberate attempt to defeat the system. Naturally such an arrangement is likely to result in numerous spurious alarms, especially in areas where commercial power is subject to interruptions. But central sta-tions almost invariably have emergency power supplies, so that the danger of loss of protection resulting from a central station power failure is virtually negligible.

One problem which is presented in providing central station power for system operation is that of power fluctuations, such as are caused by a momentary interruption of the line and subsequent restoration of the circuit. Such conditions occur from various causes, especially regular central station operating procedures, and momentary iiuctuations in line current and voltage also occur from external sources, eg., transient line current iiuctuations resulting from induction from power lines supplying power -to heavy rotating machinery. It is important that such momentary power liuctuations not result in an alarm since once an alarm condition is regis` tered by energization of relay X, the relay locks up through a holding coil X1 and the alarm will remain registered until the system is reset, as will be described hereinafter.

When an interruption in the central station line occurs or when the line Voltage drops to zero or a low value by reason of a power fluctuation, the capacitor 53 will start to discharge but the discharge current will not produce energization of relay X since the necessary power will not be available for transistor 50. When the line is closed or the line voltage returns to the normal range, a charging current will flow into capacitor 53 through rectifier 62. Butthis charging current will not produce an alarm since it will not produce a corresponding current flow through the main coil of relay X. Thus the use of a capacitor discharging current only for alarm actuation permits the supply of system power from the central station or `other remote point at which the alarm signal is to receive attention.

Of course, an open line or a drop in line voltage below the normal operating range will not go unnoticed at the central station. Thus central stations normally have sensitive instruments connected to the line to detect open circuit or low voltage conditions. Such equipment has traditionally been in the form of galvanometer type relays. A highly desirable modern type of central station line condition sensing instrument is disclosed in the copending patent application of Dominick Ghersi, Serial No. 821,290, filed June 18, 1959, now Patent 2,983,912 issued May 9, 1961.

System sensitivity adjustment System sensitivity is adjusted by changing the tap on the oscillator transformer secondary winding to which the protected object is connected. Lf the object, or more properly, the capacity to ground of the object, is connected to a lower tap on this secondary winding, the sensitivity is decreased because of the smaller coupling provided between the capacity of the object and the total capacitive circuit. By a lower tap is meant one less closely coupled physically tothe primary winding, the

tap Til-T2 being the highest and the tap 'T6-T7 the lowest.

if the sensitivity is lowered by connecting to a lower tap .the dynamic range of the system is increased and more capacitive protective can be connected to the system. A wider dynamic range means that objects connected to a lower tap can be allowed to drift more in capacity without exceeding the dynamic limit of the system, i.e., the frequency change per unit time required to energize the relayX through discharge of capacitor 53 or the total frequency change required to energize the relay X through action of the supervisory voltage.

Thus a heavy steel safe which because of its mass inherently produces a strong capacitive eld and gives good detection may be connected to a lower tap while a small tile cabinet which will havea weaker capacitive field and which tends to give poor detection may be connected to a higher tap. Connection of a safe to a lower tap means that a greater capacitive change will be required for detection while connection of a le cabinet to a higher tap means that a lesser capacitive change will be required for detection. But a safe will generally require more time, eiort and capacitive change in an att-ack than a small le cabinet, so that the detection characteristics provided by the suggested connections Will match the protection characteristics of these objects themselves.

In the arrangement illustrated, three taps are provided,

Y 'T1-T2, VTt-T5 and T6-T7, although the tap "F6-T7 is unused. More taps or fewer taps could be provided as desired. Where yonly one object is to be protected, it is desirable that it be connected to a higher or lower tap in accordance with its mass. For example, a heavy safe would'be connected to tap T6-T7, a moderate size safe to tap T4-T5, and a small size safe or iile cabinet to tap 'f1-T2. f

- Remote test Remote testing of the system integrity and operation is important, especially with a central station connection where periodic testing may be conducted in accordance with a predetermined schedule. Remotely controlled tests are usually initiated from a central station by applying so-called test battery to the line, e.g., a reverse polarity voltage or a voltage substantially higher than the usual operating value. j

In the circuit illustrated', the coil of relay Y is coupled between the low side of the night resistor 27 and low side conductor 59 through a rectiiier 67, which might be a Zener diode of the 20Zl0 type. Relay Y is selected and adjusted so that it will not be operated by the current owing therethrough due to normal line voltage. But the increased current resulting from the higher test voltage will energize relay Y. Typically the normal line voltage might be 52 volts positive at conductor 31 while the test voltage `might be 52 volts negative at conductor 31. Or the test voltage might be 78 volts.

When relay Y operates, armature 30 thereof makes with front contact 68 connecting an adjustable capacitor 69 in parallel with the capacitive alarm circuit. Operation of armature 30 disconnects capacitor 28 from the coil of relay Y, the capacitor 28 aiiording slow operation of relay Y to prevent line current surges from operating relay Y. A capacitor 69 is connected between conductors 2.1 and 59 to serve as an alternating current ground to balance the potential therebetween when armature'ai) transfers to contact 68.

Capacitor 69 is adjusted toprovidea capacity value approximately 4simulating an approach of an intruder to the protected object or objects and preferably to provide a change in frequency of the oscillator slightly in excess of the value necessary to cause an alarm.

' The change in signal frequency resulting from connection of capacitor 69 to the oscillator circuit decreases rapidly the detector voltage, resulting in energization of relay X, as described previously. With relay X energized,

armature 65 makes With front contact 66, energizing holding coil Xl of relay X thereby locking relay X in alarm condition.

The current ilowing through coil Xi also iiows through the coils of bell '76, ringing a local bell and thus signalling locally a successful test. This is useful when premises are being closed for the night since the subscriber may then leave his premises knowing that protection is being aiiorded. The bellris preferably arranged not to ring when normal voltage is on the line since it is generally considered undesirable to produce a local alarm signal indication when an attack rather than a test is detected. However, for a system without a remote connection the bell 7) can be arranged to provide -a local alarm signal indication even on normal voltage. Y

When relay X operates, the increase in line current iiow caused by closing of armature 65 and front contact 66 signals the central station a successful completion of the test. Thereupon the central station operator may momentarily open the line, allowing relays X and Y to drop out and then close the line, but with normal rather than test voltage.

Protection ojj condition When protection for the object or objects protected by the system of the invention is not desired, e.g., during business hours in a mercantile or industrial establishment, the incoming central station line current iiows through line resistor 32, cabinet lining 33, door and tamper switch 34, high side fixed attachments 35, on-oi switch 36, a day switch 71, a resistor 72, low side iixed attachments 45, and bell contact 46. The line resistor 32 affords line current adjustment. The cabinet lining 33 and door and tamper switch 34 atiord protection for the housing in which the system is contained, being arranged to open the circuit if an eti-ort is made to open the housing, thereby signalling the central station. The high and low side fixed attachments represent permanently installed protection circuits, e.g., window foils on closed windows, which provide protection in and about the premises.

Protection on condition When it is desired to provide normal protection, the

movable attachments 37 and 44 are connected, these` being protection elements which are normally disconnected during the day. The attachments 37, as shown, include a door switch which has normally closed contacts (shown closed) which open when the protected door is opened and normally open contacts which close to provide a ground on the central station line when the door is opened. These contacts, which have a common movable element, provide a so-called double drop signal when the door is opened, i.e., lirst an open or break and then a ground. When the local protection circuits are connected, the switch 71 is opened, allowing central station current to flow through the high side movable attachments 37, bridge rectier 3S, relay Y, rectifier 67, and the low side movable attachments 44.

Bridge rectifier 38 permits operation with either positive or negative voltage on line 31, being arranged in either event to provide positive voltage on conductor 58. Rectiiier 67, which is preferably of the Zener type, acts as a voltage regulator and the voltage drop thereacross, e.g., 20 volts, is available to operate the electronic circuit.

A momentary opening of the switch 36 will signal the central station that the premises are about to be vacated. The central station may thereupon initiate a test, as described above, to ascertain that the system is operating properly. The ringing of the bell 79 will afford a local indication of a successful test. The central station Will be informed of a successful test by the increased current flow occurring when relay X operates and armature 65 makes with contact 66, providing a shunt path for central station line current. To protect against possibleshunting of the low side fixed and movable attachments to ground, the bell contact 46 is connected between the low side fixed attachments and ground. If no shunting exists, when the bell operates the contact 46 will open and close the circuit at a rapid rate. lf shunting exists, the bell will not ring.

A manual switch 73 is provided in series with coil Xl to permit release of relay X under local control, as during system adjustment.

While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departing from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. An electrical protection system for signalling the approach of a foreign body to a protected object having a capacity `to ground which varies with the approach of foreign bodies, comprising an oscillator having a requency determining circuit including as a variable element the capacity .to ground of said object, a resonant circuit having a predetermined resonant frequency slightly higher than and bearing a predetermined relationship to the operating frequency of said oscillator in the absence of a foreign body in the region of said object, means to supply oscillatory energy from said oscillator to said resonant circuit whereby substantially a predetermined signal voltage is produced across at least a portion of said resonant circuit when said predetermined relationship exists and a lower signal voltage is produced across said portion of said resonant circuit when said predetermined relationship is altered by the presence of a foreign body in the region of said object, an impedance element, means coupled to said impedance element and to said resonant circuit to rectify said signal voltage and to produce across said impedance element a detector voltage whose magnitude is proportional to the magnitude of said signal voltage, a source of a supervisory voltage, means to apply said supervisory voltage across said impedance element with the polarity of said supervisory voltage being opposite lto the polarity of said detector voltage, alarm signalling apparatus, a capacitive element coupled to said impedance element, a charging circuit for said capacitive element including said impedance element whereby said capactive element is charged by said detector voltage, a discharging circuit for said capacitive element, means responsive to the current flow in said discharging circuit to operate said alarm signalling apparatus when said discharge current of said capacitive element exceeds a predetermined value, and a rectifier element connected in parallel with said capacitive clement and poled so as to pass a current flow in said discharging circuit when said detector voltage is less than said supervisory voltage thereby to operate said alarm signalling apparatus.

2. An electrical protection system for signalling the approach of a foreign body to any one or more of a plurality of protected objects each having a capacity to ground which varies with the approach of foreign bodies, comprising an electronic device having input and output circuits coupled in regenerative relationship to form an oscillator and including a transformer having a primary winding coupled to said device, said transformer having a multi-tapped secondary Winding forming a part of the frequency determining circuit of said oscillator, means to connect each of said protected objects to a respective tap of said secondary winding thereby to include the capacity to ground of said protected objects in said frequency determining circuit, connection of each of said protected objects to .a respective tap of said secondary Winding providing a respectively different degree of coupling between the capacity to ground of each of said protected objects and said frequency determining circuit whereby the relative effect of changes in the capacity to ground of said objects on the frequency of said oscillator are different for connection of said objects to different ones of said taps, means to adjust the frequency of said oscillator to a predetermined frequency value in the absence of a foreign body in the region of any one or more of said objects, a resonant circuit having a selected resonant frequency greater than said predetermined frequency value, means including an additional winding on said transformer to Supply oscillatory energy from said transformer to said resonant circuit whereby substantially a predetermined signal voltage is produced across an inductive port-ion of said resonant circuit in the absence of a foreign body in the region of any one or more of said objects and a lower signal voltage is produced across said portion of said resonant circuit in the presence of a foreign body in .the region of any one or more of said objects, means to produce an output signal having a value dependent upon both the magnitude of decrease and the time rate of decrease of said signal voltage, alarm signalling means arnanged to be operated only when said output Vsignal achieves a predetermined value, and means responsive to the magnitude of said signal voltage and independent of the time rate of change of said signal voltage to produce said output signal in at least said predetermined value when said signal Voltage falls to a predetermined level thereby to operate said alarm signalling means.

3. An electrical protect-ion system as set forth in claim 2 in which each of said protected objects has a closure member and a switch arranged to be closed when said closure member is closed and open when said closure member is open, connection of said protected objects to said respective taps being means through said switches whereby the capacity to ground of any of said protected objects is removed from said frequency determining circuit when the corresponding closure member is open.

4. An electrical protection system for signalling at a remote point the approach of a foreign body to a protected object having a capacity to ground which varies wit-h the approach of foreign bodies; comprising protection apparatus located near said protected object; signal receiving apparatus loc-ated at said remote point; an electrically conductive channel interconnect-ing said signal receiving apparatus and said protection apparatus and aording two conductive paths between said signal receiving apparatus land said protection apparatus; and means at said remote point to supply a direct current to said protection apparatus over said paths, said signal receiving apparatus being responsive to predetermined changes in said direct current to register said predetermined changes as signals from said protection apparatus; said protection apparatus comprising a rectifier circuit coupled between said paths and arranged to provide an operating potential of fixed polari-ty irrespective of the polarities of said respective paths, a first transistor amplifier, means to supply said operating potential to said lirst transistor amplifier, a transformer having a primary winding, a secondary winding and an additional winding, said first transistor amplifier having an input circuit and an output circuit intercoupled in regenerative relationship and including said primary winding, said regeneration being suliicient to cause said first transistor amplifier to oscillate, means to connect said protected object to said secondary wind-ing of said transformer whereby the capacity to ground of said protected object is effective in determining the frequency of oscillation of said first transistor amplifier, means to adjust said frequency of oscillation to a predetermined radio frequency value in the absence of a foreign body in the region of said protected object, a series resonant circuit having a selected resonant frequency greater than said predetermined frequency value, said predetermined frequency value lying wit-hin the effective range of the resonance curve of said resonant circuit whereby a signal voltage having substantially a predetermined magnitude appears across at least a portion of a reactive component of said resonant circuit in the absence of a foreign body in the region of said protected object and a signal maaar lli voltage having a magnitude less than said predetermined magnitude .appears across said portion of said reactive component of said resonant circuit When a foreign body is in the region of said protected object, an impedance element, a iirst rectifier element intercoupling said resonant circuit and said impedance element whereby a fixed polarity detector voltage proportional to the magnitude of said signal voltage appears across said impedance element, means intercoupling said rectiiier circuit and said impedance element to produce across said impedance element a supervisory voltage of fixed magnitude and having, a polarity opposite to that of said detectorvoltage, a second transistor ampliiier having an input circuit and an output circuit, means to supply said operating potential tto said second transistor amplifier, a capacitive element intercoupling said impedance element and said input circuit ot said second transistor amplifier, a second rectifier element coupled to said capacitive element and ar-V ranged to permit said detector voltage to charge said capacitive element, a third rectifier element intercoupling said impedance element and said input circuit of said second transistor amplier and being poled so as to supply current in a forward direction to the input circuit of said `second transistor amplier when the magnitude of said supervisory voltage exceeds the magnitude of said de- Itector voltage thereby to produce an output current from said second transistor amplifier greater than a predetermined value, and a relay having a coil coupled to said output circuit of said second transistor amplier and arranged to operate s-a-id relay only when the output current of said second transistor ampliiier exceeds said predetermined value thereof, said relay having contacts arranged to produce one of said predetermined changes in direct current ow in said channel whereby said signal receiving apparatus registers a signal When the output current of said secondtr-ansistor ampliier exceeds said predetermined value thereof.

5. An electrical protection system as set fort-h in claim 4 comprising an additional relay coupled to said channel and arranged to be operated upon a predetermined change Iin current flow in said channel, an additional capacitive element having a capacity value comparable to the change in capacity to ground of said protected object produced by the approach of a foreign body to the region of said protected object, and means effective upon operation of said additional relay to connect said additional capacitive element to the frequency determining circuit of said oscillator.

References Cited bythe Examiner UNITED STATES PATENTS 2,083,335 6/37 Loudon 340-258 2,086,818 7/37 Nichols et al 340-258 '2,108,202 2/38 Kelly 331-65 2,368,052 1/45 Unger 331-65 2,422,542 6/47 Gustasson 340-258 2,455,376 12/48 Lindsay 340-258 2,492,388 12/ 49 Martin 340-258 2,512,879 6/50 Roggenstein 331-65 2,971,184 2/61 Pearson et al 340-258 2,992,420 7 61 Rifken 340-276 3,041,592 6/62 Schmidt 340-258 v3,042,908 7/ 62 Pearson 340-244 FOREIGN PATENTS 353,627 7/31 England.

NEIL C. READ, Primary Examiner.

BENNETT G. MILLER, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No 3, 189, 884 June l5, 1965 Peter Laakmann It is hereby certified that error appears in the above numbered patent reqlrng correction and that the said Letters Patent should read as oorrectedbelow.

Column l0, line 3l, for "means" read made Column l2, after line 2l, insert the following reference:

Signed and sealed this 16th day of November 1965.

(SEAL) Arrest:

ERNEST W. SWIDER EDWARD J. BRENNER Allcsting Officer Commissioner of Patents 

1. AN ELECTRICAL PROTECTION SYSTEM FOR SIGNALLING THE APPROACH OF A FOREIGN BODY TO A PROTECTED OBJECT HAVING A CAPACITY TO GROUND WHICH VARIES WITH THE APPROACH OF FOREIGN BODIES, COMPRISING AN OSCILLATOR HAVING A FREQUENCY DETERMINING CIRCUIT INCLUDING AS A VARIABLE ELEMENT THE CAPACITY TO GROUND OF SAID OBJECT, A RESONANT CIRCUIT HAVING A PREDETERMINED RESONANT FREQUENCY SLIGHTLY HIGHER THAN AND BEARING A PREDETERMINED RELATIONSHIP TO THE OPERATING FREQUENCY OF SAID OSCILLATOR IN THE ABSENCE OF A FOREIGN BODY IN THE REGION OF SAID OBJECT, MEANS TO SUPPLY OSCILLATORY ENERGY FROM SAID OSCILLATOR TO SAID RESONANT CIRCUIT WHEREBY SUBSTANTIALLY A PREDETERMINED SIGNAL VOLTAGE IS PRODUCED ACROSS AT LEAST A PORTION OF SAID RESONANT CIRCUIT WHEN SAID PREDETERMINED RELATIONSHIP EXISTS AND A LOWER SIGNAL VOLTAGE IS PRODUCED ACROSS SAID PORTION OF SAID RESONANT CIRCUIT WHEN SAID PREDETERMINED RELATIONSHIP IS ALTERED BY THE PRESENCE OF A FOREIGN BODY IN THE REGION OF SAID OBJECT, AN IMPEDANCE ELEMENT, MEANS COUPLED TO SAID IMPEDANCE ELEMENT AND TO SAID RESONANT CIRCUIT TO RECTIFY SAID SIGNAL VOLTAGE AND TO PRODUCE ACROSS SAID IMPEDANCE ELEMENT A DETECTOR VOLTAGE WHOSE MAGNITUDE IS PROPORTIONAL TO THE MAGNITUDE OF SAID SIGNAL VOLTAGE, A SOURCE OF A SUPERVISORY VOLTAGE, MEANS TO APPLY SAID SUPERVISORY VOLTAGE ACROSS SAID IMPEDANCE ELEMENT WITH THE POLARITY OF SAID SUPERVISORY VOLTAGE BEING OPPOSITE TO THE POLARITY OF SAID DETECTOR VOLTAGE, ALARM SIGNALLING APPARATUS, A CAPACITIVE ELEMENT COUPLED TO SAID IMPEDANCE ELEMENT, A CHARGING CIRCUIT FOR SAID CAPACITIVE ELEMENT INCLUDING SAID IMPEDANCE ELEMENT WHEREBY SAID CAPACITIVE ELEMENT IS CHARGED BY SAID DETECTOR VOLTAGE, A DISCHARGING CIRCUIT FOR SAID CAPACITIVE ELEMENT, MEANS RESPONSIVE TO THE CURRENT FLOW IN SAID DISCHARGING CIRCUIT TO OPERATE SAID ALARM SIGNALLING APPARATUS WHEN SAID DISCHARGE CURRENT OF SAID CAPACITIVE ELEMENT EXCEEDS A PREDETERMINED VALUE, AND A RECTIFIER ELEMENT CONNECTED IN PARALLEL WITH SAID CAPACITIVE ELEMENT AND POLED SO AS TO PASS A CURRENT FLOW IN SAID DISCHARGING CIRCUIT WHEN SAID DETECTOR VOLTAGE IS LESS THAN SAID SUPERVISORY VOLTAGE THEREBY TO OPERATE SAID ALARM SIGNALLING APPARATUS. 